Bug 13757: (QA followup) Make opac-memberentry.pl handle attrs deletion

Bug 13757: (QA followup) Make opac-memberentry.pl handle attrs deletion

The original code on this bug skipped empty-valued attributes. But
emptying attributes is the only way to tell the controller script that
the user wants to delete them.

This patch makes opac-memberentry.pl check the existence of attributes
sharing the code of the empty for the given patron, and it stores the
deletion on the Koha::Patron::Modification as needed. Otherwise
deletions got skipped.

To test:
- Verify setting/deleting attributes that are opac-editable and verify
the results are sound.

https://bugs.koha-community.org/show_bug.cgi?id=13737

Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>